My personal opinion? Unless you know that the exact device you want is coming out in a known period of time that you don't mind waiting (like, a device coming out in May 09 and you can wait that long), I'd just get what you like out there now. It's a growing technology, and like every other technology there's always something newer and better coming out.
That being said, I love my 505 and haven't gotten jealous of other new ebook readers yet. But I know other 505 owners who have turned to the Dark Side
The 700 might be a better choice for you, FYI, since you want to read PDFs and annotate, and is about the same size as the 505. I know there is another device that reads PDFs and annotates, but I'm woefully uneducated on it, so I'm sure someone else will pipe in
